A typical process for the aqueous dispersion polymerization of fluorinated monomer includes feeding fluorinated monomer to a heated reactor containing a fluorosurfactant and deionized water. Paraffin wax is employed in the reactor as a stabilizer for some polymerizations, e.g., polytetrafluoroethylene (PTFE) homopolymers. A free-radical initiator solution is employed and, as the polymerization proceeds, additional fluorinated monomer is added to maintain the pressure. A chain transfer agent is employed in the polymerization of some polymers, e.g., melt-processable TFE copolymers, to control melt viscosity. After several hours, the feeds are stopped, the reactor is vented and purged with nitrogen, and the raw dispersion in the vessel is transferred to a cooling vessel.
For use in fluoropolymer coatings for metals, glass and fabric, polymer dispersion is typically transferred to a dispersion concentration operation which produces stabilized dispersions used as coatings. Certain grades of PTFE dispersion are made for the production of fine powder. For this use, the polymer dispersion is coagulated, the aqueous medium is removed and the PTFE is dried to produce fine powder. Dispersions of melt-processable fluoropolymers for molding resin are also coagulated and the coagulated polymer dried and then processed into a convenient form such as flake, chip or pellet for use in subsequent melt-processing operations.
As described in U.S. Pat. No. 3,391,099 to Punderson, dispersion polymerization involves two generally distinct phases. The initial period of the reaction is a nucleation phase in which a given number of polymerization sites or nuclei are established. Subsequently, there occurs a growth phase in which polymerization of fluorinated monomer on established particles occurs with little or no formation of new particles. Successful production of the high solids fluoropolymer dispersion generally requires the presence of the fluorosurfactant, especially in the later growth phase of polymerization in order to stabilize the dispersion preventing coagulation of the fluoropolymer particles.
Fluorosurfactants used in the polymerization are usually anionic, non-telogenic, soluble in water and stable to reaction conditions. The most widely used fluorosurfactants are perfluoroalkane carboxylic acids and salts as disclosed in U.S. Pat. No. 2,559,752 to Berry, specifically perfluorooctanoic acid and salts, often referred to as C8, and perfluorononanoic acid and salts, often referred to as C9. Because of recent environmental concerns with regard to perfluorooctanoic acid and salts, there is interest in reducing or eliminating perfluorooctanoic acid and salts in fluoropolymer polymerization processes.
High molecular weight materials such as fluoropolyethers have been used in the polymerization of fluoromonomers. U.S. Pat. No. 4,864,006 to Gianetti et al. discloses the polymerization of fluorinated monomers in the presence of a perfluoropolyether having neutral end groups, perfluoropolyether oil, which is used in the form of an aqueous microemulsion. The perfluoropolyether oil has a molecular weight of at least about 500 and the aqueous microemulsion of the oil is prepared using a suitable surfactant which can be selected from known perfluorinated carboxylic or sulfonic acids or from perfluoropolyethers having one or two acid end groups. U.S. Pat. No. 3,271,341 to Garrison discloses a process that uses perfluoropolyether carboxylic acid or salt as dispersing agents in the aqueous polymerization of fluoromonomers. U.S. Pat. No. 6,395,848 to Morgan et al. discloses an improved process for the aqueous dispersion polymerization of fluorinated monomers using a combination of fluorosurfactants, which can be a fluoroalkyl carboxylic or sulfonic acid or salt thereof or fluoroalkoxy aryl sulfonic acid or salt thereof, and perfluoropolyether carboxylic or sulfonic acid or salt thereof. U.S. Pat. No. 7,141,537 to Audenaert et al. also discloses mixtures of fluorinated polyether surfactants and their use as an emulsifying agent or stabilizer in dispersion polymerization.
U.S. Pat. No. 3,271,341 to Garrison focuses on the use of a number of individual perfluoroether dispersing agents of varying degree of polymerization where n is a value of the number of repeating ether units and is disclosed as 0 to 10. Such dispersing agents are produced from the polymerization of tetrafluoroethylene oxide or hexafluoropropylene oxide by contacting the oxide with a typical free radical-forming catalyst which results in a perfluoropolyether carbonyl fluoride which is converted into an acid, ester or alcohol. The reaction product of the polymerization of hexafluoropropylene oxide is a mixture of perfluoropolyether acids or salts of varying degree of polymerization containing in one specific example, a broad molecular weight distribution of about 12% of a fraction of n=0 (MW of about 330 g/mol), about 5% of a fraction of n=1 (MW of about 500 g/mol), about 50% of a fraction of n=2-35 (MW of from about 660 g/mol to about 6140 g/mol) and about 8% of a fraction n>35 (MW of above about 6140 g/mol). Isolated individual acids from this mixture are used in polymerization in Garrison's Examples.
U.S. Pat. No. 6,395,848 (Morgan et al.) illustrates the use of mixtures of perfluoropolyether acids or salts having a broad range of molecular weight fractions characterized by number average molecular weights ranging from 2000 to 7500 g/mol, for example Krytox® FSL 157 with number average molecular weight of 2500 g/mol and Krytox® FSH 157 with number average molecular weight of 7000-7500 g/mol. U.S. Pat. No. 7,141,537 to Audenaert et al. illustrates perfluoropolyether surfactants having a broad range of molecular weight fractions characterized by weight average molecular weights of from 750 g/mol to 5000 g/mol. Fluoropolyether acids or salts with broad molecular weight ranges including high molecular weight fractions can result in residual high molecular weight fluoropolyether acids or salts and/or by-products in the resulting fluoropolymer which may be undesirable for some end uses.
It would be desirable to have an improved mixture of fluoropolyether acids or salts thereof which is particularly useful for commercial fluoropolymer manufacture and which controls residual fluoropolyether acids or salts and by-products in the resulting polymer product.